To me, the forces that end Arthur's reign are the result of his own greatness. In other words, the same forces that make him great are the ones that end up casting him down.

I see two reasons for Arthur's ultimate defeat. First is the fact that he fathered Mordred. Second is his desire for revenge. He loses his throne to Mordred because he is out seeking vengeance against Launcelot for his affair with Guinevere.

What these have in common, in my opinion, is that they are both the result of Arthur's great and immoderate drive. His lust for sex causes the birth of Mordred and his lust for personal honor and revenge opens the door for Mordred to depose him.

So I think that Arthur's immoderate drives make him great but also lead to his downfall.